1. Field of the Invention
The present invention relates to a connectionless communication system and a method of connection communication, and more particularly to a large capacity connectionless communication system and a method of large capacity connectionless communication, capable of achieving an application to both a public network and a private network by centralizing all connectionless data and carrying out a routing of the centralized connectionless data by use of a separate connectionless cell switching unit having a rapid connection setup function and a function for maintaining the set-up connection.
2. Description of the Prior Art
Generally, traffics transferred via a network are classified into the connection type, wherein a frame transfer is achieved under a condition of setting up a communication path between a transmitter and a receiver by a connection setup procedure using, for example, telephones, and the connectionless type, wherein a frame transfer is achieved on the basis of a destination address included in a header of data frame without any connection setup procedure, as in a local area network (LAN).
An asynchronous transfer mode (ATM) network is of the connection type adapted to transfer frames to a destination necessarily using a connection setup procedure. In order to transfer connectionless data used in a LAN to a destination in such an ATM, a frame transfer method must be used, which method involves a connection setup achieved using a virtual channel previously set up or using an destination address value of every connectionless frame every time when the connectionless frame is generated.
In order to realize supply of connectionless data services in the ATM network, International Telecommunication Union (ITU) recommendation I.327 has proposed a method of directly connecting terminals to communicate with each other via a virtual channel and a method of using connectionless servers for achieving a routing function for connectionless data cells.
As the method of exchanging connectionless data through the virtual channel connected between terminals, there have been known a method of setting up a semi-permanent or permanent virtual channel between terminals and transferring data via the set-up virtual channel, and a method of transferring connectionless data under a condition that a connection has been set up using a destination address value of every connectionless frame every time when the connectionless frame is generated.
In the case of using the semi-permanent or permanent virtual channel, however, all of virtual channels associated with all terminals which are to communicate should be previously set up. As a result, the number of virtual channels to be set up is abruptly increased as the number of terminals to communicate is large. Such an increased number of virtual channels results in a complicated network management and an inefficient use of network resource. For this reason, this method can be applied only to the case where the number of terminals is small (Reference: N. Kavak, LAN interconnection over B-ISDN (ATM), Interworking'92, Swiss, 1992).
On the other hand, the latter method of setting up the virtual connection every time when the connectionless frame is generated involves a very large transfer delay caused by the connection setup. This is because the method involves not only setting up the virtual connection every time when the connectionless frame is generated, but also releasing the set-up connection after transfer of the frame. Furthermore, an overload is subjected to a switching system because of frequent connection setup and release procedures. As a result, this method can not be applied to a public network. The application of this method is limited to a small scale private network requiring simplified connection setup and release procedures.
In case of an ATM switching system, it requires a connection setup procedure for setting up a virtual connection. However, the connection setup procedure requires a very high time as compared to a transfer delay of cells. This results in a delay caused by connection setup and release of terminals generating connectionless traffic and an overload of the ATM switching system caused by frequent connection set up and release procedures. For this reason, such an ATM switching system uses typically a method of setting up a semi-permanent virtual connection and transferring connectionless cells via the set-up virtual connection.
In order to achieve a communication between LANs through an ATM network, it is necessary to set up a virtual channel on the basis of addresses of the LANs. To this end, the address of the ATM should be searched utilizing the addresses of LANs. An address resolution protocol (ARP) is also required (Reference: Akira Chugo, Kazuo Sakagawa, Broadband communication network architecture for distributed computing environments, IEICE Trans. Comm., Japan, March 1994).
In the case of the method of using connectionless servers, the network resource may be efficiently utilized by centralizing all connectionless data generated from terminals on corresponding connectionless servers via virtual channels each set-up between each of all terminals generating the connectionless data and each corresponding to one of the connectionless servers and routing the centralized data on the basis of destination address values of frames respectively in the servers (Reference: K. Kerkhof, A. Halderen, AAL type 5 to support the broadband connectionless data bearer service, Interworking'92, Swiss, 1992 and N. Kavak, LAN interconnection over B-ISDN (ATM), Interworking'92, Swiss, 1992).
In this method using connectionless servers, however, the connectionless data transferred in the form of cells is recovered in the form of original frames in each server so as to determine a destination on the basis of a destination address value of each frame. To this end, this method requires processing of a connectionless network access protocol (CLNAP) layer adapted to determine a destination on the basis of a destination address value of each recovered frame. As a result, the servers have a complex function. Consequently, it is difficult to realize connectionless servers of a large capacity. Therefore, this method has a problem that a bottle neck occurs at connectionless servers because all connectionless data are processed in a centralized manner.
A new method using a rapid connection setup has also been proposed in order to prevent an occurrence of the necking phenomenon and yet efficiently use the network resource. However, this method is inappropriate for connectionless services requiring setup of a new connection and release thereof for every frame. This is because the ATM switching unit construction requiring mapping of virtual path identifiers (VPIs)/virtual channel identifiers (VCIs), routing, update of a header translation table has a limitation on the rapid connection setup.
A scheme has also been made to take only the advantages of both the method using permanent virtual channels and the method using the rapid connection setup. This scheme is to setup a connection by using the rapid connection setup method and maintain the set-up connection by use of a cash. This method is effective for a private ATM-LAN requiring a small scale connection of terminals. However, this method has the disadvantages of a strict management of bandwidths of all terminal connections and a difficult application to a public network requiring a large scale connection of terminals.
Meanwhile, there has also been proposed a method of setting up a partial connection between a transmitting terminal and an ATM switch connected to a destination terminal and transferring connectionless cells via the set-up partial connection (Relating Patent "Support to connectionless services in ATM networking using partial connections" filed on Feb. 27, 1992 and bearing EPU Application No. 92810298.7, and Reference "J. Y. Boudec, H. L. Truong, Providing MAC services on an ATM network with point-to-point links", Interworking'92, Swiss, 1992). In accordance with this method, setup of a partial connection is achieved by a management function in response to a request of a terminal. Once the partial connection is set up, VPI/VCI values of the set-up connection is transferred to the terminal so as to generate a function for maintaining the set-up connection.
However, this method can not be applied to a typical ATM network complying with a standard recommended by the ITU. This is because the ATM switch, to which destination terminals are connected, uses VPI/VCI regions for cells divided into partial connection identifier (PCI), message multiplexing (MMX) identifier, connectionless (CL)/connection oriented (CO) identifier and end of message (EOM) identifier so that it may transmit cells received from a plurality of partial connections to receiving terminals by multiplexing them. Moreover, this method still has the problem of the delay caused by the connection setup.
A LAN traffic transfer method has also been proposed for an application to a case wherein a plurality of bridges respectively connected with LANs are connected to an ATM network. In accordance with this method, transfer of LAN traffics is accomplished through a virtual connection with a full-mesh structure set-up between associated bridges (Relating Patent "Network system with ATM network LAN bridge" filed on Aug. 27, 1990 and bearing Patent No. EP-473066). Used in this method is a local address filter table (L-AFT) in which each bridge stores addresses of terminals respectively connected with LANs associated therewith in a learned manner and a remote address filter table (R-AFT) in which each bridge stores addresses of terminals associated with other bridges and virtual connection numbers for the bridges in a learned manner. Accordingly, a virtual connection for transmitting cells is selected on the basis of contents of the tables.
For an application to a case wherein a plurality of LANs are connected to an ATM network constituted by a plurality of ATM switches, there has also been proposed a method of performing transfer of connectionless traffic from LANs in an ATM switching unit. In accordance with this method, once a frame from a LAN reaches one of the ATM switches, the ATM switch carries the destination address value of the frame to all LANs by use of a control cell to broadcast the destination address value of the frame. Of ATM switches receiving the control cell, the one associated with a terminal corresponding to the destination address transfers a response cell to the ATM switch which has transmitted the control cell. Upon receiving the response cell, the ATM switch which has transmitted the control cell transfers the frame to the received response cell.
However, both the methods mentioned just above propose only a method for selecting one direction to the destination from virtual connections. They have no disclosure relating to a solution to the problem involved in the large scale LAN connection or relating to an accomplishment of the large capacity.
On the other hand, there has been proposed a method wherein a connectionless data transmitting terminal requests a routing identifier (RI) value corresponding to a destination address in accordance with a routing identifier management function (RIMF) so that the RI value may be transmitted by a cell which carries the RI value on its VCI region. In accordance with this method, the RI value includes a switch node identifier (ID) and a port ID directing to a destination switching node. On the basis of this RI value, a routing is carried out. Each of the ATM switches has a VPI/VCI table for achieving a routing of connection cells and a header translation and an RI allocation table for achieving a routing of connectionless cells and a conversion of the RI value. This means that connection and connectionless cells should be processed differently depending on the kind or type thereof. For this reason, the kind or type of each cell should be indicated on the VPI region of the cell or other cell region.
As a result, an ATM switching unit operating in accordance with this method should have a function for processing connection and connectionless cells differently depending on the kind of cell. In accordance with this method, the setup of virtual connections for transfer of connectionless cells is carried out in a manner approximately similar to the setup of virtual connections for transfer of connection cells. Consequently, this method still has the problem of the delay caused by the connection setup.